Method for making a glass-to-metal seal



June 2, 1970 R. LANDRON, JR

METHOD FOR MAKING A GLASS-TO-METAL SEAL Original Filed Dec. 31, 1964 2 Sheets-Sheet 1 P8 o m 80- M &3 70- 8a to 60- 02 31 50- t 2:; ml 38 30- 3 o 20- GLASS-SEALING ALLOY H N0 CONCENTRATION (NORMALS) A (END! Rafael Landr0n,Jr.

INVENTOR.

June 2, 1970 R. LANDRON, JR 3,514,849

METHOD FOR MAKING A GLASS-TO-METAL SEAL Original Filed Dec. 31, 1964 2 Sheets-Sheet 2 Rafael Landron Jr.

INVENTOIi United States Patent 3,514,849 METHOD FOR MAKING A GLASS-TO-METAL SEAL Rafael Landron, Jr., Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Original application Dec. 31, 1964, Ser. No. 422,801. Divided and this application Nov. 15, 1968, Ser. No. 798,523

Int. Cl. H011 N US. Cl. 29-588 5 Claims ABSTRACT OF THE DISCLOSURE Disclosed is a method for fabricating a semiconductor device utilizing the selective dissolution of copper fro-m a glass sealing alloy by exposing the copper and the alloy to HNO solution.

This is a division of application Ser. No. 422,801, filed Dec. 31, 1964.

This invention relates to a method of chemically separating one metal from another by the selective dissolution effect of exposing different metals to a particular concentration of acid. More specifically, it relates to the separation of a discrete quantity of copper from a glass-sealing metal alloy by exposing the two metals to a particular concentration of HNO Even more specifically, it relates to a method for fabricating a glassto-metal seal in a metal header, upon which header an electronic component, for example a semiconductor wafer, can be mounted.

The material most often used in packages for semiconductor devices is a metal alloy known under the trade name Kovar. This alloy is particularly useful due to its glass-sealing properties, this being due to the fact that the alloy is wet by molten glass and has the same temperature expansion coefiicient as certain hard glasses. Compositions for this alloy vary slightly, examples being 54% iron28% nickel-48% cobalt, and nickel- 17% cobalt--0.2% manganesebalance iron. The material will be identified herein as an iron-nickel-cobalt glass-sealing alloy.

It has been common practice in the manufacture of some glass-to-metal seals, for example those used in the fabrication of integrated circuits, to use a weighted graphite plug to force the molten glass into the desired areas, such as around the metal leads, and also to shape the cavity to accommodate the semiconductor wafer. The graphite plug is the source of many problems, because once the glass has fused around it, the plug can only be removed by sandblasting or by some similar means. The sandblasting operation is expensive, requiring individual handling and a high degree of operator skill, since excessive sandblasting results in some of the glass being removed from the seal and causing the header to thus become defective. The graphite plug also will cause bubbles to form in the glass of some of the seals, thus causing even more of the headers to be defective.

It is therefore the principal object of the invention to provide a method of fabricating a glass-to-metal seal in an eflicient and economical manner.

Another object is to provide a method of selectively removing one metal from another metal by exposing both metals to an acid solution.

Another object is to provide a method of fabricating a glass-to-metal seal which is substantially free from bubbles in the glass.

Still another object is to provide a method of fabricating a semiconductor electronic device.

Patented June 2, 1970 "ice Other objects, features, and advantages of the invention will become apparent from the following description of the preferred embodiment when taken in connection with the appended claims and attached drawing, wherein like numerals represent like parts, in which:

FIG. 1 illustrates the relative solubility of copper and Kovar compared with a varying concentration of HNO FIGS. 2a and 2b illustrate sectional views of a header according to the preferred embodiment of the invention.

FIG. 3 illustrates a cut-away pictorial view of a semiconductor device fabricated according to the invention.

Since a plug of some different material than graphite was deemed necessary, investigations of several materials resulted in copper being the ultimate choice for the plug. This was not immediately satisfactory, however, because a solid copper plug produces bubbles in the glass in a manner similar to that of graphite. The bubble problem was eliminated by making the plug porous, for example by the well-known powder metallurgy method, including underfiring of the plugs.

Another problem connected with the use of copper as a plug was that there was no known solvent which would dissolve the copper plug and yet not damage the glass-sealing metal alloy used for the frame of the header. An investigation of several different solvents showed that HNO at concentrations of greater than about 10.5 N, preferably 12 N or higher, would dissolve copper without dissolving Kovar which is the glasssealing metal alloy ordinarily used. The investigation also showed, illustrated in FIG. 1, that a very low concentration of HNO approximately 1.5 N or lower, would dissolve the preferred iron-nickel-cobalt glasssealing alloy but not copper, while some intermediate point, approximately 9 N, would dissolve both the glasssealing alloy and copper.

Another problem connected with using copper as the plug was that copper adheres quite strongly to the glass seal and also to the metal weights. This problem was overcome by immersing the plug in a liquid graphite suspension and subsequently drying it in a 250 C. oven for approximately 30 minutes. The graphite coating thus obtained was found not to cause bubbles as did solid graphite, and eliminated the adhesion problem of copper to glass and copper to the metal weights.

Because of the high thermal coefficient of expansion of copper, approximately 15 X10 cm./cm./ C, compared with glass approximately 8X10" cm./cm./ C, the copper contracts more than the glass and actually pulls away from it during the cooling process. Thus it was observed that some of the plugs do not require the HNO treatment because the different thermal coefficients of expansion effect a separation of the plug from the rest of the header. For those devices which do not separate upon cooling, the HNO bath is the next step, and as the acid works its way under the copper plug, the plug will be separated even before it is completely dissolved.

Referring now to FIG. 2 a sectional view of the preferred embodiment built according to the invention is illustrated. FIG. 2(A) shows the graphite fusing jig 3 and 4 clamped over the Kovar leads ti. Jig 4 has a Kovar base plate 5, upon which a portion of some sealing glass 8A, for example Corning 7052, is placed. The graphite-coated, porous copper plug 9 is then placed upon the glass. The metal weight 1 is then placed upon the copper plug 9 and is guided by the weight guide 2. The Kovar header ring frame 7 is in a position within the fusing jig to complete the glass-to-metal seal assembly once the fusion step, as illustrated in FIG. 2(B), is effected. As further shown in 3 FIG. 2A, 8B represents a space to be later occupied by the glass 8A. The whole assembly, as shown in FIG. 2A, is then placed in a fusing furnace (not shown) and heated to approximately 1000 C. FIG. 23 illustrates how the molten glass 8 is pressed by the plug 9 and Weight 1 around the leads 6 and Kovar frame 7 to complete the glass-to-metal seal. The fusing jig 3 and 4 is then removed (merely unclamped), weight 1 is lifted from the copper plug 9, and the header, which includes the glass 8, the leads 6 and the Kovar ring frame 7 is then allowed to cool, along with the plug 9. As the copper of the plug 9 contracts during cooling, it pulls away from the glass 8 in some cases. In those instances Where the plug 9 does not pull away, the header and plug is then immersed n HNO of a concentration of 12 N or higher until the acid undermines the plug enough to cause separation of the plug from the glass or until the plug is completely dissolved, whichever is desired.

Still another advantage of this method of removing the copper is that once the concentration of HNO is at least 12 N, the solution may be used until the acid is deleted. Thes is contrary to expectations since it would be expected that as the acid becomes weaker, the Kovar would begin to dissolve. However, it was found that the products of the reaction between copper and HNO somehow inhibit the reaction with Kovar. The over-all reaction between HNO and copper is as follows:

This unexpected result of being able to use the acid Subsequent to the copper plug 9 of FIG. 2 being removed,

it is seen that the copper plug 9 causes the glass 8 to have an indented area 14. Area 14 thus serves as a mounting surface for a semiconductor wafer 10. The electrical lead wires 12 connect contact lands on the semiconductor wafer 10 to the pads 11, the pads being the interiorly exposed portion of leads- 6. Notches 13 in the metal alloy frame 7 allow leads to pass through the frame 7 and when the glass 8 is fused, the notches are filled with the glass and a glass-to-metal seal is efliected between the leads '6, the glass 8 and the frame 7. It should be appreciated that the semiconductor wafer or wafers thus mounted may be any combination of N- or P-type materials and may be composed of germanium, silicon, gallium arsenide or the like and would have any number of transistors, resistors, etc., formed therein.

The frame 7 extends to the base plate 5 so that, if desired, a top plate 15 may be welded to the frame with the weld current passing vertically through the frame. Also, the plate 15 may be welded to the frame with electrodes which engage only the plate and the side of the frame so that weld current through the base plate is unnecessary.

The depression 14 in the center of the glass 8 may ex- 4 tend all the way to the base plate 5 so that the semiconductor wafer 10 may rest directly upon the base plate.

Although the invention has been described with reference to a specific preferred embodiment, it is understood that modifications and substitutions, such as other processes requiring either the separation of a glass-sealing metal alloy, such as Kovar, from copper or copper from such an alloy can be made without departing from the scope thereof as defined by the appended claims.

What is claimed is:

1. A method for fabricating a semiconductor device comprising the steps of (a) positioning a plurality of inwardly extending flat leads about and above the periphery of a flat base plate, the leads and base plate being composed of a first metal;

(b) positioning a ring composed of said first metal in spaced relation to said leads;

(c) placing a mass of sealing glass at approximately the center of said base plate;

(d) placing a plug on said mass, the plug being substantially composed of a second metal;

(e) placing a weight on said plug;

(f) heating the assembly to fuse said glass whereby the fused glass will flow to effect a seal between said flat leads, said base plate and said ring;

(g) removing said plug from the assembly by subjecting the assembly to an etching solution such that the plug will be dissolved while the ring, base plate and leads remain substantially intact;

(h)1 mounting a semiconductor wafer above said base p ate;

(i) attaching a plurality of lead wires from said semiconductor wafer to said flat leads, and

(j) attaching a top metallic plate to said assembly, whereby said device becomes hermetically sealed.

2. The method according to claim 1 wherein said first metal is comprised of an iron-nickel-cobalt alloy, and said second metal is substantially copper.

3. The method according to claim 2 wherein said etching solution is HNO having a concentration of at least approximately 10.5 normals.

4. The method according to claim 2 wherein said copper plug is porous.

5. The method according to claim 4 wherein said porous copper plug is graphite coated.

References Cited UNITED STATES PATENTS 2,897,419 7/1959 Howland et al. 29-588 3,212,161 10/ 1965 Oxley 29588 3,271,124 9/1966 Clark.

3,381,369 5/1968 Stoller.

PAUL M. COHEN, Primary Examiner US. Cl. X.R. 29423, 472.9, 504 

